Nanotechnology (NT) collectively refers to a technology of manufacturing, manipulating or analyzing matter on a nanometer scale. Nanotechnology may be largely classified into synthesis of nanomaterials and application of them. Especially, porous materials have been traditionally used for catalysts, absorbents, support materials, or the like due to their high surface area. These materials are classified into microporous, mesoporous and macroporous materials according to the pore size.
In general, a nanoporous material refers to the microporous material and the mesoporous material. Until the early 1990, zeolite was the representative inorganic microporous material. With a pore size of 2 nm or smaller, zeolite is also called the molecular sieve. Due to a peculiar network structure consisting of aluminum, silicon and oxygen atoms, zeolite can serve as a solid acid and an ion exchanger. Therefore, it has been usefully utilized in many applications. For example, it can be used to selectively adsorb or separate molecules suiting the pore size, and has been used for cracking and reforming reactions in the petrochemical industry due to its specific acid sites or by supporting of catalytically active materials. Recently, it is also used as a support for various nanomaterials due to its nano-sized pores. However, since the pore size of zeolite is relatively small, its application for larger molecules is limited. Hence, it has been a challenge to synthesize a material which has the properties of zeolite but a larger pore size.
In 1992, Mobil announced the synthesis of M41S mesoporous silica families including MCM-41 and MCM-48 (MCM stands for Mobil Composition of Matter). As shown in the electron microscopic image of FIG. 1, these materials have very uniform nanometer-sized pores, which are arranged regularly like the honeycomb. Most importantly, the pore size can be tuned between 2 nm and 30 nm by varying the synthesis conditions. The synthesis of these materials became a turning point in the nanopore research, and mesoporous materials of various structures were synthesized by many researchers afterward. For example, SBA-15, which is synthesized from an ethylene oxide (hydrophilic)-propylene oxide (hydrophobic) block copolymer as a template material under an acidic condition (pH=1-2), has a pore size of 5-30 nm and is known to be hydrothermally stable, with a relatively large silica wall thickness (ca. 3-6 nm). HMS (standing for hexagonal mesoporous silica) is synthesized using a neutral surfactant such as alkylethylene oxide (BRIJ). Spherical silica is bound to the surfactant through hydrogen bonding. MSU (standing for Michigan State University) is synthesized under a neutral condition, using polyethylene oxide (PEO) as a biodegradable, non-ionic, neutral surfactant. MCM-41 and MCM-48 may be synthesized under a basic condition using a cationic surfactant such as cetyltrimethylammonium bromide (CTAB) as a template material.
Furthermore, researches about the use of these mesoporous materials as catalyst and for other applications are also increasing. Especially, researches are actively carried out in the field of nanochemistry, nanotechnology, supramolecular chemistry, or the like.
These mesostructures may be formed by controlled packing of micelles or inverse micelles of various structures which are prepared from a variety of surfactants and amorphous silica. Alkoxysilanes such as tetramethyl orthosilicate (TMOS), tetraethyl orthosilicate (TEOS) are usually employed as a silica source.
Recently, attempts have been made to replace the organosilicates, which are relatively expensive, produce waste during the manufacturing process, and are potentially environmentally harmful themselves, with silicon dioxide, natural silicate materials, kanemite, water glass, silica gel, or the like as a silica precursor, which are naturally occurring and produce less waste.